1. Field of the Invention
The present invention relates to an optical compensatory sheet employable for compensating phase difference of a liquid crystal cell, and a liquid crystal display provided with the optical compensatory sheet.
2. Description of Prior Art
As a display for electronic office system-devices such as a desk-top personal computer and a word processor for Japanese language, CRT (cathode ray tube) has been employed so far. Recently, a liquid crystal display (hereinafter referred to as LCD) is increasingly employed instead of the CRT because of its thin thickness, light weight and low power consumption. LCD generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. Most of LCD use a twisted nematic liquid crystal or a super twisted nematic liquid crystal.
A super twisted nematic liquid crystal display (hereinafter referred to as STN-LCD) has been recently utilized for dot-matrix type liquid crystal displays such as those of word processors and personal computers. The liquid crystal display generally has a structure that a liquid crystal cell is disposed between a pair of polarizing sheets. The STN-LCD uses a super twisted nematic liquid crystal showing a twisted angle of 180 to 270 degrees. Such STN-LCD has an advantage of showing a high contrast on high multiplexing drive, compared with the conventional twisted nematic liquid crystal display (twisted angle: 90 degrees) which can be utilized for the dot-matrix type liquid crystal displays.
However, the STN-LCD has a disadvantage of giving a displayed image inherently colored with the hue from blue or yellow owing to elliptically polarized light transmitted through STN liquid crystal cell. This phenomenon may be hereinafter referred to as coloring. In the STN-LCD, high contrast cannot be obtained in black-and-white display and it is difficult to obtain color image.
In order to avoid the coloring of the displayed image, there have been proposed a liquid crystal display having one or three layers of optically anisotropic body (that is, NTN mode-display) and a liquid crystal display using a uniaxial stretched polymer film (that is, FTN mode-display).
The former NTN mode-display shows excellent characteristics in black-and-white display. The liquid crystal cell for optically anisotropic body used in the NTN mode-display is voluminous and a plurality of such cells are required, so that the NTN mode-display is large in volume and weight, and high in production cost compared with the FTN mode-display using a uniaxial stretched polymer film. Hence, the uniaxial stretched polymer film is mainly used as the optical compensatory film.
Various polymer films (optical compensatory films (birefringence films)), for example a uniaxial stretched polycarbonate film, have been developed for the purpose of the elimination of coloring of the STN-LCD. Thus, the coloring is reduced and the display provided with the film shows almost a black-and-white image.
The above liquid crystal display provided the uniaxial stretched film has been improved in elimination of coloring, so far as the coloring when the display is viewed from the direction vertical to the screen is concerned. However, when the liquid crystal display is viewed from an oblique direction, unfavorable viewing angle characteristics such as coloring and lowering of contrast of a displayed image and reversing of black-and-white image are observed.
To improve the above viewing angle characteristics, a method of employing both of a film of positive intrinsic birefringence and a film of negative intrinsic birefringence or employing their composite, and method of employing merely a film of negative intrinsic birefringence have been proposed. These methods, however, scarcely give improvement of the viewing angle characteristics.
Japanese Patent Provisional Publication No. 2(1990)-285330 discloses that a birefringence film wherein a refractive index in a thickness direction is larger than that in a direction perpendicular to an optic axis of birefringence is employed as the optical compensatory sheet. Such sheet satisfies the condition of nx&gt;nz&gt;ny wherein nx, ny and nz represent main refractive indices. The use of the birefringence film brings about enhancement of contrast to some extent when the display is viewed from an oblique direction. However, the sheet does not satisfactorily improve the viewing characteristics.
Further, optical compensatory sheets having oblique optic axis or oblique nz (thickness direction) have been proposed (Japanese Patent Provisional Publications No. 4(1992)-120512, No. 4(1992)-113301, No. 5(1993)-80323 and No. 5(1993)-157913).
Japanese Patent Provisional Publication No. 4(1992)-120512 discloses an optical sheet whose polymer chain is polarized in the direction at a certain angle to the plane of the sheet. This sheet is considered to satisfy nx&gt;nz =ny or nx=nz&gt;ny and have oblique optic axis (that only the direction of nz is inclined). Japanese Patent Provisional Publication No. 4(1992)-113301 discloses an optical sheet whose polymer has an average oriented direction arranged at a certain angle to the plane of the sheet. Japanese Patent Provisional Publication No. 5(1993)-80323 discloses an optical sheet whose optic axis is inclined to the plane of the sheet. These two sheets are prepared by obliquely slicing a uniaxial polycarbonate film, and therefore the sheets are considered to satisfy nx&gt;nz=ny and to have oblique optic axis (in which all three directions of nx, nz and ny are inclined).
Japanese Patent Provisional Publication No. 5(1993)-157913 also discloses an optical sheet that the direction of nz is inclined from the plane of the sheet. Further, the sheet satisfies the condition of nx&gt;nz&gt;ny.
The above optical compensatory sheets having oblique optic axis also do not improve satisfactorily the viewing angle characteristics such as coloring and contrast of a displayed image, and reversing of black-and-white image, when the liquid crystal display is viewed from an oblique direction.
When the viewing direction to the liquid crystal display is inclined from the normal to a surface of the display, quality of the displayed image depends upon not only (nx-ny).multidot.d (=Re; retardation (d:thickness)) but also (nx-nz).multidot.d or (nz-ny).multidot.d. The sheet satisfying the condition of nx&gt;nz&gt;ny (Japanese Patent Provisional Publication No. 2(1990)-285330) is designed in consideration of the above relationship of retardation. The sheet decreases luminance of black displayed portion, when the display is viewed from an oblique direction, in normally black mode that is widely adopted in STN-LCD.
In the sheet, tilt angle produced by orientation of liquid crystal in a liquid crystal cell is not considered. Therefore, the viewing angle characteristics such as coloring and contrast of a displayed image and reversing of black-and-white image, when the display is viewed from an oblique direction, are not satisfactorily improved. It is reported that the tilt angle of liquid crystal used for STN-LCD brings about asymmetry of optimum Re in all viewing directions (The 37th Applied Physics Society, 30a-D-10, Spring, 1990).
As is described above, the optical compensatory sheets having oblique optic axis or oblique nz (thickness direction) are also proposed. Especially, Japanese Patent Provisional Publication No. 5(1993)-157913 discloses an optical sheet whose optic axis is inclined from the plane of the sheet and which satisfies the condition of nx&gt;nz&gt;ny. However, the sheet does not satisfactorily improve the viewing angle characteristics.